Applications such as electrospinning and printing require highly mobile fluids as they are often passed through small orifices with diameters at micrometers or less scale. Electrospinning uses electrical forces to produce polymer fibers with nanometer-scale diameters. Electrospinning occurs when the electrical forces at the surface of a polymer solution or melt overcome the surface tension and cause an electrically charged jet to be ejected. Having carbon nanotubes in the fluid could aid in development of electrical forces and uniformity of the voltage gradient for improved printability. It is also highly desirable to have concentrated yet mobile fluids of carbon nanotubes for efficient dispersion into materials such as lead oxide or silicon particles that are can be made into pastes for energy storage or collection devices such as batteries, capacitors and photovoltaics. Yet another need for concentrated yet mobile fluids of carbon nanotubes is for the formation of uniform thin electrical conducting coatings, for example in displays and sensors. The coating thicknesses are often at the micrometer scale or below and require higher concentrations of solids to reduce drying times and hence lower cost of manufacturing.
A challenge with creating mobile fluids with high solids content wherein the solid is a rod-like structure is that at certain concentrations the rods can interact with each other. This is called the percolation threshold concentration. Studies have shown that the volume concentration percolation threshold, V, generally follows the equation V=0.6/(L/D), where L is the length and D is the diameter of the rod. An example of a reference to this equation is “Simulations and electrical conductivity of percolated networks of finite rods with various degrees of axial alignment” S. I. White B. A. DiDonna, M. Mu, T. C. Lubensky and K. I. Winey. Department of Materials Science & Engineering Departmental Papers (MSE), University of Pennsylvania Year 2009. The ratio L/D is also called the aspect ratio of the rod. Above the percolation threshold concentration the properties of the composite show a change in performance, such as large increases in viscosity. For example, if the interactions of the rods are sufficient, gels can form. The nature of the interactions between rods can be, for example, mechanical or electrostatic. An example of mechanical interaction is where flexible rods entangle.
Carbon nanotubes can be classified by the number of walls in the tube, single wall, double wall and multiwall. Each wall of a carbon nanotube can be further classified into chiral or non-chiral forms. Carbon nanotubes are currently manufactured as agglomerated nanotube balls or bundles. They are well known to have good electrical and thermal properties.
PCT US 2011/0294013A1 has disclosed exfoliated carbon nanotubes, methods of production and products thereof. The terms “exfoliated” or “discrete” are used to indicate that the carbon nanotubes are untangled from clusters or associated bundles of carbon nanotubes that are the result of their original manufacture using, for example, catalysts in a gas phase reactor. During the process of making discrete carbon nanotubes (which can be in single, double and multiwall configurations), the nanotubes are cut into segments and optionally functionalized. The cutting of the tubes reduces the length of the tubes into carbon nanotube segments that are defined here as Molecular Rebar.
Various methods have been developed to unbundle carbon nanotubes in solution. For example, carbon nanotubes may be extensively shortened by oxidative means and then dispersed in dilute solution. Concentrations of carbon nanotubes in the solutions are often less than 0.1 percent weight/volume. Carbon nanotubes may also be dispersed in solution as individuals by sonication in the presence of a surfactant. Illustrative surfactants used for dispersing carbon nanotubes in solution include, for example, sodium dodecyl sulfate and block polymers such as polyethylene oxide-polypropylene oxide polymers sold under the Pluronic® trademark by BASF. In some instances, solutions of individualized carbon nanotubes may be prepared from polymer-wrapped carbon nanotubes. Individualized single-wall carbon nanotube solutions have also been prepared using polysaccharides, polypeptides, water-soluble polymers, nucleic acids, DNA, polynucleotides, polyimides, and polyvinyl pyrrolidone. Disclosed in U.S. Pat. No. 7,682,590 B2 are carbon nanotubes dispersed in polar organic solvent and methods for producing the same. In this disclosure, single wall carbon nanotubes taken directly from the high pressure carbon monoxide process without being oxidized are dispersed at concentrations of 0.01% by weight using non-ionic surfactants in n-methylpyrrolidone and furthermore, filtered using filters with porosity 0.1 to 3 micrometers. The resultant more dilute filtrate mixture of single wall carbon nanotubes is reportedly stable and does not form visible aggregates or settle.
In each of these methods of unbundling carbon nanotubes in fluid media, such as water, the concentration of carbon nanotubes in the fluid medium is below their percolation threshold volume concentration. Since carbon nanotubes can have lengths of many micrometers, yet be 1-50 nanometers in diameter, this means that percolation threshold concentrations can be as little as fractions of a percentage by volume. It is therefore highly desirable to obtain fluids of carbon nanotubes of solids content above the percolation threshold concentration as determined by the length and diameter of the carbon nanotubes for applications requiring maximum content carbon nanotubes and minimum content solvent, or maximum carbon nanotube content and minimum fluid viscosity. One example of an application is the addition of discrete carbon nanotubes to lead oxide paste where the allowable water content is restricted and there is a need for a high concentration of discrete carbon nanotubes within the lead oxide paste . . . .
In some applications, for example printing inks and coatings, the fluid containing concentrations of discrete carbon nanotubes above the percolation threshold can be dried to form a film in which the discrete carbon nanotubes collapse upon each other into a dense network of tubes and surfactant. The film is non-dustable and conductive. The film, being a polymer encapsulated collection of discrete carbon nanotubes, provides reduced safety concerns for users, with minimal inhalation risks. The drying process which creates this film is identical to that observed during printing of inks; the printed ink will also be un-dustable, adhering well to paper and other substrates, with a minimal chance of discrete carbon nanotubes being released to the air or environment. The higher concentration of discrete carbon nanotubes in the mobile fluid allows such processes as, but not limited to, ink-jet printing and faster drying.